The present invention relates to a hole cutter or core drill for cutting a relatively large hole in a relatively thick plate of metal such as steel.
The applicant's U.S. Pat. No. 4,408,935 dated Oct. 11, 1983 shows a metal borer of this general type including a shank 4, a cylindrical blade 1 and a centering pin 8, which is supported by the shank and urged by a spring to normally project forwardly from the blade. When in use, first, a thick metal plate has a small center recess formed by a separate drill, and the pointed end of pin 8 is located in the recess. Then, the shank 4 is rotated and pressed forwardly so that the blade 1 cuts a hole around the recess, while the pin 8 is retracted into the shank 4. If the center recess is too shallow or the spring is too weak, the pin 8 may jump out of the recess during the operation, thus losing the centering action.
The applicant's Japanese Patent Early Publication No. H.4-105,811 dated Apr. 7, 1992 shows another hole cutter including a shank 2, a cylindrical blade 6 and a center drill 10, which is fixed to the shank 2 and projects forwardly from the blade 6. First, the drill 10 bores a center bore in a metal plate. If the plate is thick, the center boring may continue after the blade 6 begins cutting, and consequently add a resistance to the cutting resistance of the blade 6, thus lowering the operation efficiency.
It is a general object of the present invention to provide a hole cutter having both a center drill for initially boring a sufficiently deep center bore in a metal plate, and a centering pin for insertion into the bore.